A. Field of the Invention
This invention relates to piezoelectric transducers, and more particularly, to a means and method for nonuniform poling of piezoelectric transducers.
B. Problems in the Art
Piezoelectric transducers are utilized for a wide variety of applications. A few examples are ultrasonic holography, acoustic imaging, and nondestructive evaluation using electromagnetic transduction such as ultrasound. These uses and advantages of piezoelectric transducers are well known in the art.
A piezoelectric transducer used, for example, with nondestructive evaluation ultrasonic inspection, generally consists of a disk of piezoelectric ceramic material (an example of which is referred to as PZT) connected to electrical leads and mounted in a housing. The disk is prepared for use by poling, that is, setting the polarization throughout the disk. This means that electrical energy is passed through the disk for a period of time. The disk is then installed within the transducer case or housing and is electrically connected to circuitry.
Operation of piezoelectric ultrasonic transducers is well known in the art. To transmit ultrasonic energy, a timedependent electrical voltage is supplied to the piezoelectric disk. This causes the disk to vibrate, which in turn produces high frequency sound waves.
The transducer can also serve as a receiver. The piezoelectric disk would receive sound energy, vibrate in kind, and in turn produce a small electrical signal proportional to received sound energy. This signal can be amplified for further use.
The process of poling of the disk, that is, producing an initial polarization in the disk before utilizing the disk in the transducer, is critical to how the transducer operates. Normally, the disk is uniformly poled. What this means is that a uniform electrical field is applied across the disk by attaching electrodes on opposite sides of the disk. A conductive layer is spread across both sides of the disks so that the electrical potential proposedly spreads out evenly through the disk.
The way the piezoelectric element is poled determines its response pattern or profile. This is important in achieving reliable and interpretable results using such transducer.
For example, use of these types of transducers in ultrasonic nondestructive evaluation requires that the ultrasonic beam pattern be generally known both so that the results obtained by monitoring the reflection of the ultrasonic energy can be accurately and reliably determined; and also to allow the ultrasonic energy to have a beam profile or pattern which is as effective as possible in analyzing materials for flaws (e.g. eliminate disadvantageous beam characteristics such as near field oscillations or side lobes; reduce or eliminate diffraction of the beam; or create a beam whose reflections can be more accurately analyzed). Conventional uniformly poled piezoelectric transducers produce an approximately uniform field across their face. While they may be easy to fabricate, the field patterns produced by uniformly poled transducers are complicated.
For example, the field patterns of uniformly poled piezoelectric transducers have large amplitude oscillations in the near field. There are also side lobes in the beam profile. Even though these types of patterns are well known in the art, they produce complexities and disadvantages in such applications as nondestructive evaluation.
Examples of these problems are as follows. The large oscillations in the near field on-axis pressure contain nulls. Flaws located at the near field nulls can therefore easily escape detection. Secondly, in quantitative nondestructive evaluation, the complicated beam profile from uniformly poled transducers requires complicated analytical techniques. As a practical matter, these complications are significant enough to result in substantially more processing time and steps than if a simpler or different beam pattern was available.
It is important to understand that the disadvantage of the complicated analysis required for uniformly poled transducer beams is substantial and significant. To derive relevant and meaningful information from ultrasonic evaluation, tremendous amounts of computing and processing time are needed. This requires dedication of a certain significant amount of computing power, and of course, requires substantial time. Any reduction in these requirements is beneficial. Elimination of detrimental beam profiles shapes and utilization of advantageous beam profile shapes can result in substantial, if not staggering reductions in processing computation time. In some cases, this reduction can be as great or greater than 1,000 fold.
Another example of a problem in the art is that conventional ultrasonic beams from uniformly poled transducers, like most energy beams, experience significant diffraction or beam spreading over distance. Thus, the effective range of such beams is limited. Diffraction must also be taken into consideration in analyzing the beam or any reflections of the beam for purposes of such things as ultrasonic testing or ultrasonic nondestructive evaluation.
If diffraction of ultrasonic beams could be reduced or eliminated, effective range of the beam would be significantly improved, and many additional uses for ultrasonic evaluation would be enhanced. New applications might also be developed. It has been reported that by altering the beam profile of electromagnetic waves into a form defined by a Bessel function, diffraction of the beam can be significantly reduced or eliminated. If similar beam-shaping could be applied to ultrasound transducers, it may be possible to eliminate diffraction problems with acoustic waves. Uniformly poled transducers exhibit these diffraction problems.
It can therefore be seen how a uniformly poled transducer can be deficient in certain applications. A need exists, therefore, for transducers which did not exhibit the problems and deficiencies of a uniformly poled transducer.
It has been determined that if the transducer could be made to vibrate in such a way that the amplitude profile at the face of the transducer followed a Gaussian function, being a maximum at the center and falling off like a Gaussian function toward the perimeter, the beam profile would be extremely simple, which would solve the above mentioned problems. The Gaussian shaped beam would not have oscillations in its axial amplitude and would not have side lobes. Furthermore, the transverse profile of a Gaussian beam is described by a Gaussian function at any distance from the transducer.
As is well known in the art, a Gaussian function is approximately a bell-shaped curve when graphed. Attempts have then been made to produce such a piezoelectric transducer so that it generates what would be called a Gaussian beam.
Most of these attempts involve utilizing a uniformly poled piezoelectric transducer disk, but operating the disk by a nonuniform driving voltage and field. In other words, preparation and initial poling of the disk would be by conventional methods resulting in a uniformly poled disk. However, the electrical voltage utilized to drive the transducer would be presented to the uniformly poled disk in a nonuniform way; and particularly, approximating that of a Gaussian.
One such method utilized an eight-pointed star-shaped electrode on one face of the uniformly poled transducer disk or element. The driving voltage and field presented through that star-shaped electrode approximated a Gaussian function. This method is described in K. V. Haselberg and J. Krautkramer, "Ein Ultraschall-Strahler fur die Werkstoffprufung mit Verbessertem Nahfeld", Acustica 9, 359-364 (1959).
Others produced approximate Gaussian beams by utilizing a full electrode plating on one face of the uniformly poled transducer element, but used a small electrode with a diameter equal to about three times the thickness on the other face. See F. D. Martin and M. A. Breazeale, "A Simple Way to Eliminate Diffraction Lobes Emitted by Ultrasonic Transducers", J. Acoust. Soc. Am. 49, 1668-1669 (1971) and G. Du and M. A. Breazeale, "Ultrasonic Field of a Gaussian Transducer", J. Acoust. Soc. Am. 78, 2083-2086 (1985). Other attempts utilized electrodes positioned on the uniformly poled disk made of concentric rings each driven by a different voltage provided by a voltage divider network. See P. S. Zerwekh and R. O. Claus, "/Ultrasonic Transducer with Gaussian Radial Velocity Distribution", in Proc. 1981 IEEE Ultrason. Symp., Cat. No. 81CH1689-9; and R. 0. Claus and P. S. Zerwekh, "Ultrasonic Transducer with a Two-Dimensional Gaussian Field Profile", IEEE Trans. Sonics and Ultrasonics, 30, 36-39 (1983).
These methods did produce approximate Gaussian transducer response profiles to eliminate near field oscillations and diffraction side lobes. However, all of these methods require the additional structure and complexity of means and methods to provide nonuniform driving voltages and fields to a uniformly poled transducer element.
A more recent attempt to provide a Gaussian function piezoelectric transducer can be found at U.S. Pat. No. 4,518,889 to T Hoen. In this approach, a matrix of parallel rods of piezoelectric ceramic is configured to operate as the transducer element. Different polarizations to different regions of that composite element are then produced. The T Hoen patent also briefly alleges that the flat non-uniformly poled piezoelectric element can be produced by abutting a block of material to one of the element faces. The block of material has electrical properties such as resistivity and dielectric constant which effectively form a voltage divider. Electricity is then passed through the block of material and the flat element. This patent then alleges (without giving evidence) the polarization of the flat PZT element will be nonuniform, and can be Gaussian, based on the geometry and configuration of the block of material.
The T Hoen patent therefore also has a similar deficiency of requiring rather complex structure to produce a nonuniform poled transducer element. The parallel rods do not make a continuous transducer surface. Additionally, the above-described alternative method utilizing the block of material is not believed to be functional. The high dielectric constant of piezoelectric ceramic makes it impossible to polarize the piezoelectric element because almost all the voltage will be dropped across the gap between such a block of material and the piezoelectric disk.
It can therefore be seen that there is a real need in the art for a nonuniformly poled piezoelectric transducer. Not only is there a need to be able to create transducers which do not exhibit the deficiencies of uniformly poled transducers, but there is also a need to be able to selectively incorporate nonuniform poling of a variety of different beam profiles for various applications. Still further, there is a need to be able to produce nonuniformly poled transducers which do not require complex driving voltages or apparatus to provide nonuniform driving voltages, or specially configured composite transducer elements. There is a real need to have a nonuniformly poled transducer which can be incorporated into standard conventional transducer probes.
It is therefore a primary object of the present invention to provide a means and method for nonuniform poling of piezoelectric transducers which solves or improves over the problems and deficiencies in the art.
A further object of the present invention is to provide a means and method as above described which allows different poling or field profiles to be introduced into piezoelectric transducer elements.
A further object of the present invention is to provide a means and method as above described which allows specific functions such as Gaussian or Bessel functions to be incorporated into the response and beam profiles of transducer elements.
Another object of the present invention is to provide a means and method as above described which allows elimination of the near field nulls and side lobes in the response patterns, if desired, of uniformly poled transducers.
Another object of the present invention is to provide a means and method as above described which allows creation of response and beam profiles of desired mathematical functions.
A still further object of the present invention is to provide a means and method as above described which minimizes diffraction or beam spreading of the beam, if desired, to reduce beam spreading which would diminish dissipation of the beam or distance it can penetrate.
Another object of the present invention is to provide a means and method as above described which allows generation of specific beam patterns such as Bessel function or Gaussian function patterns.
Another object of the present invention is to provide a means and method as above described wherein the transducer has a large range if desired.
A further object of the present invention is to provide a means and method as above described which allows beam width and frequency to be varied independently, if desired.
A further object of the present invention is to provide a means and method as above described which allows production of a nonuniformly poled piezoelectric transducer which is of the same physical structure and dimensions as a conventional uniformly poled element so that it can be directly incorporated into existing transducer probes without modification.
Another object of the present invention is to provide a means and method as above described which is efficient, economical, and reliable.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.